The torque of a synchronous motor is proportional to the angular difference between the magnetic field of the rotor and the magnetic field of the stator, i.e. to the rotor angle. The torque is at a maximum when the rotor angle is 90° and decreases according to a sine function as the rotor angle changes. The torque curve of synchronous motors designed for use in elevator drives is a nearly sinusoidal function of the rotor angle. One of the tasks of the elevator control system is to keep the torque at the maximum point.
At present, the rotor position is typically determined by means of a resolver, which produces feedback data on the absolute rotor position, which is needed e.g. in vector control.
However, flat elevator motors designed for elevators without machine room and placed in the elevator shaft provide relatively little space, which is why it is often not possible to use a resolver in such a situation because it can not be mounted in the elevator shaft due to insufficient space. In addition, a resolver is relatively expensive and adjusting it is a complicated task.
To determine the rotor position data, it is also possible to use a pulse emitter or a tachometer. However, such systems are relatively device-dependent and are therefore not directly applicable e.g. for use in elevators. The pulse emitter or tachometer is generally connected to the rotor via a belt transmission or a friction wheel. This involves a slip, which tends to increase. Thus, there is the risk that the torque will decrease. The motor may even fall out of synchronism, in which case the torque is lost completely.